The present invention relates generally to serial line isolators, and more specifically, to a method and apparatus for isolating faults in emergency input and notification circuits.
In emergency systems, notification and input devices are powered through the use of input device circuits (IDCs) and notification appliance circuits (NACs). These circuits include several different zones, each containing one or more devices. Input devices include such devices as smoke detectors; and notification devices include such devices as sirens and strobe lights. IDCs and NACs have generally been set up such that the devices are connected to one another in series. Due to this series configuration, a fault in any given zone or device will affect all other zones or devices in the circuit. Regulations require that a fault in one zone does not affect the operation of other zones.
Traditionally, line isolators have been implemented on the notification and input circuits in order to isolate faults in any given zone. These line isolators have been implemented in a series configuration, such that each isolator is connected in series with each zone. If a short circuit fault, such as a wire-to-wire short, is detected in the zone, the line isolators on each end of the zone open, isolating the zone from the rest of the circuit. Power is then supplied on a return path in order to continue to power the devices in the zones which are further down the line from the newly opened isolator.
This past configuration works for isolating a single fault, but does not work to isolate multiple faults in a circuit, or to handle open circuit faults. If a fault occurs in a first zone, and then another fault occurs in a second zone further down the line, there is no way for the system to continue to provide power to any of the intermediate zones. Thus, there is a need to be able to isolate multiple faults in an emergency system without losing operation of any functional zone or device.